pg_visibility.c

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/*-------------------------------------------------------------------------
 *
 * pg_visibility.c
 *    display visibility map information and page-level visibility bits
 *
 * Copyright (c) 2016-2020, PostgreSQL Global Development Group
 *
 *    contrib/pg_visibility/pg_visibility.c
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/visibilitymap.h"
#include "catalog/pg_type.h"
#include "catalog/storage_xlog.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/procarray.h"
#include "storage/smgr.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"

PG_MODULE_MAGIC;

typedef struct vbits
{
    BlockNumber next;
    BlockNumber count;
    uint8       bits[FLEXIBLE_ARRAY_MEMBER];
} vbits;

typedef struct corrupt_items
{
    BlockNumber next;
    BlockNumber count;
    ItemPointer tids;
} corrupt_items;

PG_FUNCTION_INFO_V1(pg_visibility_map);
PG_FUNCTION_INFO_V1(pg_visibility_map_rel);
PG_FUNCTION_INFO_V1(pg_visibility);
PG_FUNCTION_INFO_V1(pg_visibility_rel);
PG_FUNCTION_INFO_V1(pg_visibility_map_summary);
PG_FUNCTION_INFO_V1(pg_check_frozen);
PG_FUNCTION_INFO_V1(pg_check_visible);
PG_FUNCTION_INFO_V1(pg_truncate_visibility_map);

static TupleDesc pg_visibility_tupdesc(bool include_blkno, bool include_pd);
static vbits *collect_visibility_data(Oid relid, bool include_pd);
static corrupt_items *collect_corrupt_items(Oid relid, bool all_visible,
                                            bool all_frozen);
static void record_corrupt_item(corrupt_items *items, ItemPointer tid);
static bool tuple_all_visible(HeapTuple tup, TransactionId OldestXmin,
                              Buffer buffer);
static void check_relation_relkind(Relation rel);

/*
 * Visibility map information for a single block of a relation.
 *
 * Note: the VM code will silently return zeroes for pages past the end
 * of the map, so we allow probes up to MaxBlockNumber regardless of the
 * actual relation size.
 */
Datum
pg_visibility_map(PG_FUNCTION_ARGS)
{
    Oid         relid = PG_GETARG_OID(0);
    int64       blkno = PG_GETARG_INT64(1);
    int32       mapbits;
    Relation    rel;
    Buffer      vmbuffer = InvalidBuffer;
    TupleDesc   tupdesc;
    Datum       values[2];
    bool        nulls[2];

    rel = relation_open(relid, AccessShareLock);

    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);

    if (blkno < 0 || blkno > MaxBlockNumber)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid block number")));

    tupdesc = pg_visibility_tupdesc(false, false);
    MemSet(nulls, 0, sizeof(nulls));

    mapbits = (int32) visibilitymap_get_status(rel, blkno, &vmbuffer);
    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);
    values[0] = BoolGetDatum((mapbits & VISIBILITYMAP_ALL_VISIBLE) != 0);
    values[1] = BoolGetDatum((mapbits & VISIBILITYMAP_ALL_FROZEN) != 0);

    relation_close(rel, AccessShareLock);

    PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
}

/*
 * Visibility map information for a single block of a relation, plus the
 * page-level information for the same block.
 */
Datum
pg_visibility(PG_FUNCTION_ARGS)
{
    Oid         relid = PG_GETARG_OID(0);
    int64       blkno = PG_GETARG_INT64(1);
    int32       mapbits;
    Relation    rel;
    Buffer      vmbuffer = InvalidBuffer;
    Buffer      buffer;
    Page        page;
    TupleDesc   tupdesc;
    Datum       values[3];
    bool        nulls[3];

    rel = relation_open(relid, AccessShareLock);

    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);

    if (blkno < 0 || blkno > MaxBlockNumber)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid block number")));

    tupdesc = pg_visibility_tupdesc(false, true);
    MemSet(nulls, 0, sizeof(nulls));

    mapbits = (int32) visibilitymap_get_status(rel, blkno, &vmbuffer);
    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);
    values[0] = BoolGetDatum((mapbits & VISIBILITYMAP_ALL_VISIBLE) != 0);
    values[1] = BoolGetDatum((mapbits & VISIBILITYMAP_ALL_FROZEN) != 0);

    /* Here we have to explicitly check rel size ... */
    if (blkno < RelationGetNumberOfBlocks(rel))
    {
        buffer = ReadBuffer(rel, blkno);
        LockBuffer(buffer, BUFFER_LOCK_SHARE);

        page = BufferGetPage(buffer);
        values[2] = BoolGetDatum(PageIsAllVisible(page));

        UnlockReleaseBuffer(buffer);
    }
    else
    {
        /* As with the vismap, silently return 0 for pages past EOF */
        values[2] = BoolGetDatum(false);
    }

    relation_close(rel, AccessShareLock);

    PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
}

/*
 * Visibility map information for every block in a relation.
 */
Datum
pg_visibility_map_rel(PG_FUNCTION_ARGS)
{
    FuncCallContext *funcctx;
    vbits      *info;

    if (SRF_IS_FIRSTCALL())
    {
        Oid         relid = PG_GETARG_OID(0);
        MemoryContext oldcontext;

        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
        funcctx->tuple_desc = pg_visibility_tupdesc(true, false);
        /* collect_visibility_data will verify the relkind */
        funcctx->user_fctx = collect_visibility_data(relid, false);
        MemoryContextSwitchTo(oldcontext);
    }

    funcctx = SRF_PERCALL_SETUP();
    info = (vbits *) funcctx->user_fctx;

    if (info->next < info->count)
    {
        Datum       values[3];
        bool        nulls[3];
        HeapTuple   tuple;

        MemSet(nulls, 0, sizeof(nulls));
        values[0] = Int64GetDatum(info->next);
        values[1] = BoolGetDatum((info->bits[info->next] & (1 << 0)) != 0);
        values[2] = BoolGetDatum((info->bits[info->next] & (1 << 1)) != 0);
        info->next++;

        tuple = heap_form_tuple(funcctx->tuple_desc, values, nulls);
        SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(tuple));
    }

    SRF_RETURN_DONE(funcctx);
}

/*
 * Visibility map information for every block in a relation, plus the page
 * level information for each block.
 */
Datum
pg_visibility_rel(PG_FUNCTION_ARGS)
{
    FuncCallContext *funcctx;
    vbits      *info;

    if (SRF_IS_FIRSTCALL())
    {
        Oid         relid = PG_GETARG_OID(0);
        MemoryContext oldcontext;

        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
        funcctx->tuple_desc = pg_visibility_tupdesc(true, true);
        /* collect_visibility_data will verify the relkind */
        funcctx->user_fctx = collect_visibility_data(relid, true);
        MemoryContextSwitchTo(oldcontext);
    }

    funcctx = SRF_PERCALL_SETUP();
    info = (vbits *) funcctx->user_fctx;

    if (info->next < info->count)
    {
        Datum       values[4];
        bool        nulls[4];
        HeapTuple   tuple;

        MemSet(nulls, 0, sizeof(nulls));
        values[0] = Int64GetDatum(info->next);
        values[1] = BoolGetDatum((info->bits[info->next] & (1 << 0)) != 0);
        values[2] = BoolGetDatum((info->bits[info->next] & (1 << 1)) != 0);
        values[3] = BoolGetDatum((info->bits[info->next] & (1 << 2)) != 0);
        info->next++;

        tuple = heap_form_tuple(funcctx->tuple_desc, values, nulls);
        SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(tuple));
    }

    SRF_RETURN_DONE(funcctx);
}

/*
 * Count the number of all-visible and all-frozen pages in the visibility
 * map for a particular relation.
 */
Datum
pg_visibility_map_summary(PG_FUNCTION_ARGS)
{
    Oid         relid = PG_GETARG_OID(0);
    Relation    rel;
    BlockNumber nblocks;
    BlockNumber blkno;
    Buffer      vmbuffer = InvalidBuffer;
    int64       all_visible = 0;
    int64       all_frozen = 0;
    TupleDesc   tupdesc;
    Datum       values[2];
    bool        nulls[2];

    rel = relation_open(relid, AccessShareLock);

    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);

    nblocks = RelationGetNumberOfBlocks(rel);

    for (blkno = 0; blkno < nblocks; ++blkno)
    {
        int32       mapbits;

        /* Make sure we are interruptible. */
        CHECK_FOR_INTERRUPTS();

        /* Get map info. */
        mapbits = (int32) visibilitymap_get_status(rel, blkno, &vmbuffer);
        if ((mapbits & VISIBILITYMAP_ALL_VISIBLE) != 0)
            ++all_visible;
        if ((mapbits & VISIBILITYMAP_ALL_FROZEN) != 0)
            ++all_frozen;
    }

    /* Clean up. */
    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);
    relation_close(rel, AccessShareLock);

    tupdesc = CreateTemplateTupleDesc(2);
    TupleDescInitEntry(tupdesc, (AttrNumber) 1, "all_visible", INT8OID, -1, 0);
    TupleDescInitEntry(tupdesc, (AttrNumber) 2, "all_frozen", INT8OID, -1, 0);
    tupdesc = BlessTupleDesc(tupdesc);

    MemSet(nulls, 0, sizeof(nulls));
    values[0] = Int64GetDatum(all_visible);
    values[1] = Int64GetDatum(all_frozen);

    PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
}

/*
 * Return the TIDs of non-frozen tuples present in pages marked all-frozen
 * in the visibility map.  We hope no one will ever find any, but there could
 * be bugs, database corruption, etc.
 */
Datum
pg_check_frozen(PG_FUNCTION_ARGS)
{
    FuncCallContext *funcctx;
    corrupt_items *items;

    if (SRF_IS_FIRSTCALL())
    {
        Oid         relid = PG_GETARG_OID(0);
        MemoryContext oldcontext;

        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
        /* collect_corrupt_items will verify the relkind */
        funcctx->user_fctx = collect_corrupt_items(relid, false, true);
        MemoryContextSwitchTo(oldcontext);
    }

    funcctx = SRF_PERCALL_SETUP();
    items = (corrupt_items *) funcctx->user_fctx;

    if (items->next < items->count)
        SRF_RETURN_NEXT(funcctx, PointerGetDatum(&items->tids[items->next++]));

    SRF_RETURN_DONE(funcctx);
}

/*
 * Return the TIDs of not-all-visible tuples in pages marked all-visible
 * in the visibility map.  We hope no one will ever find any, but there could
 * be bugs, database corruption, etc.
 */
Datum
pg_check_visible(PG_FUNCTION_ARGS)
{
    FuncCallContext *funcctx;
    corrupt_items *items;

    if (SRF_IS_FIRSTCALL())
    {
        Oid         relid = PG_GETARG_OID(0);
        MemoryContext oldcontext;

        funcctx = SRF_FIRSTCALL_INIT();
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
        /* collect_corrupt_items will verify the relkind */
        funcctx->user_fctx = collect_corrupt_items(relid, true, false);
        MemoryContextSwitchTo(oldcontext);
    }

    funcctx = SRF_PERCALL_SETUP();
    items = (corrupt_items *) funcctx->user_fctx;

    if (items->next < items->count)
        SRF_RETURN_NEXT(funcctx, PointerGetDatum(&items->tids[items->next++]));

    SRF_RETURN_DONE(funcctx);
}

/*
 * Remove the visibility map fork for a relation.  If there turn out to be
 * any bugs in the visibility map code that require rebuilding the VM, this
 * provides users with a way to do it that is cleaner than shutting down the
 * server and removing files by hand.
 *
 * This is a cut-down version of RelationTruncate.
 */
Datum
pg_truncate_visibility_map(PG_FUNCTION_ARGS)
{
    Oid         relid = PG_GETARG_OID(0);
    Relation    rel;
    ForkNumber  fork;
    BlockNumber block;

    rel = relation_open(relid, AccessExclusiveLock);

    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);

    RelationOpenSmgr(rel);
    rel->rd_smgr->smgr_vm_nblocks = InvalidBlockNumber;

    block = visibilitymap_prepare_truncate(rel, 0);
    if (BlockNumberIsValid(block))
    {
        fork = VISIBILITYMAP_FORKNUM;
        smgrtruncate(rel->rd_smgr, &fork, 1, &block);
    }

    if (RelationNeedsWAL(rel))
    {
        xl_smgr_truncate xlrec;

        xlrec.blkno = 0;
        xlrec.rnode = rel->rd_node;
        xlrec.flags = SMGR_TRUNCATE_VM;

        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, sizeof(xlrec));

        XLogInsert(RM_SMGR_ID, XLOG_SMGR_TRUNCATE | XLR_SPECIAL_REL_UPDATE);
    }

    /*
     * Release the lock right away, not at commit time.
     *
     * It would be a problem to release the lock prior to commit if this
     * truncate operation sends any transactional invalidation messages. Other
     * backends would potentially be able to lock the relation without
     * processing them in the window of time between when we release the lock
     * here and when we sent the messages at our eventual commit.  However,
     * we're currently only sending a non-transactional smgr invalidation,
     * which will have been posted to shared memory immediately from within
     * smgr_truncate.  Therefore, there should be no race here.
     *
     * The reason why it's desirable to release the lock early here is because
     * of the possibility that someone will need to use this to blow away many
     * visibility map forks at once.  If we can't release the lock until
     * commit time, the transaction doing this will accumulate
     * AccessExclusiveLocks on all of those relations at the same time, which
     * is undesirable. However, if this turns out to be unsafe we may have no
     * choice...
     */
    relation_close(rel, AccessExclusiveLock);

    /* Nothing to return. */
    PG_RETURN_VOID();
}

/*
 * Helper function to construct whichever TupleDesc we need for a particular
 * call.
 */
static TupleDesc
pg_visibility_tupdesc(bool include_blkno, bool include_pd)
{
    TupleDesc   tupdesc;
    AttrNumber  maxattr = 2;
    AttrNumber  a = 0;

    if (include_blkno)
        ++maxattr;
    if (include_pd)
        ++maxattr;
    tupdesc = CreateTemplateTupleDesc(maxattr);
    if (include_blkno)
        TupleDescInitEntry(tupdesc, ++a, "blkno", INT8OID, -1, 0);
    TupleDescInitEntry(tupdesc, ++a, "all_visible", BOOLOID, -1, 0);
    TupleDescInitEntry(tupdesc, ++a, "all_frozen", BOOLOID, -1, 0);
    if (include_pd)
        TupleDescInitEntry(tupdesc, ++a, "pd_all_visible", BOOLOID, -1, 0);
    Assert(a == maxattr);

    return BlessTupleDesc(tupdesc);
}

/*
 * Collect visibility data about a relation.
 *
 * Checks relkind of relid and will throw an error if the relation does not
 * have a VM.
 */
static vbits *
collect_visibility_data(Oid relid, bool include_pd)
{
    Relation    rel;
    BlockNumber nblocks;
    vbits      *info;
    BlockNumber blkno;
    Buffer      vmbuffer = InvalidBuffer;
    BufferAccessStrategy bstrategy = GetAccessStrategy(BAS_BULKREAD);

    rel = relation_open(relid, AccessShareLock);

    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);

    nblocks = RelationGetNumberOfBlocks(rel);
    info = palloc0(offsetof(vbits, bits) + nblocks);
    info->next = 0;
    info->count = nblocks;

    for (blkno = 0; blkno < nblocks; ++blkno)
    {
        int32       mapbits;

        /* Make sure we are interruptible. */
        CHECK_FOR_INTERRUPTS();

        /* Get map info. */
        mapbits = (int32) visibilitymap_get_status(rel, blkno, &vmbuffer);
        if ((mapbits & VISIBILITYMAP_ALL_VISIBLE) != 0)
            info->bits[blkno] |= (1 << 0);
        if ((mapbits & VISIBILITYMAP_ALL_FROZEN) != 0)
            info->bits[blkno] |= (1 << 1);

        /*
         * Page-level data requires reading every block, so only get it if the
         * caller needs it.  Use a buffer access strategy, too, to prevent
         * cache-trashing.
         */
        if (include_pd)
        {
            Buffer      buffer;
            Page        page;

            buffer = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
                                        bstrategy);
            LockBuffer(buffer, BUFFER_LOCK_SHARE);

            page = BufferGetPage(buffer);
            if (PageIsAllVisible(page))
                info->bits[blkno] |= (1 << 2);

            UnlockReleaseBuffer(buffer);
        }
    }

    /* Clean up. */
    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);
    relation_close(rel, AccessShareLock);

    return info;
}

/*
 * Returns a list of items whose visibility map information does not match
 * the status of the tuples on the page.
 *
 * If all_visible is passed as true, this will include all items which are
 * on pages marked as all-visible in the visibility map but which do not
 * seem to in fact be all-visible.
 *
 * If all_frozen is passed as true, this will include all items which are
 * on pages marked as all-frozen but which do not seem to in fact be frozen.
 *
 * Checks relkind of relid and will throw an error if the relation does not
 * have a VM.
 */
static corrupt_items *
collect_corrupt_items(Oid relid, bool all_visible, bool all_frozen)
{
    Relation    rel;
    BlockNumber nblocks;
    corrupt_items *items;
    BlockNumber blkno;
    Buffer      vmbuffer = InvalidBuffer;
    BufferAccessStrategy bstrategy = GetAccessStrategy(BAS_BULKREAD);
    TransactionId OldestXmin = InvalidTransactionId;

    if (all_visible)
    {
        /* Don't pass rel; that will fail in recovery. */
        OldestXmin = GetOldestXmin(NULL, PROCARRAY_FLAGS_VACUUM);
    }

    rel = relation_open(relid, AccessShareLock);

    /* Only some relkinds have a visibility map */
    check_relation_relkind(rel);

    nblocks = RelationGetNumberOfBlocks(rel);

    /*
     * Guess an initial array size. We don't expect many corrupted tuples, so
     * start with a small array.  This function uses the "next" field to track
     * the next offset where we can store an item (which is the same thing as
     * the number of items found so far) and the "count" field to track the
     * number of entries allocated.  We'll repurpose these fields before
     * returning.
     */
    items = palloc0(sizeof(corrupt_items));
    items->next = 0;
    items->count = 64;
    items->tids = palloc(items->count * sizeof(ItemPointerData));

    /* Loop over every block in the relation. */
    for (blkno = 0; blkno < nblocks; ++blkno)
    {
        bool        check_frozen = false;
        bool        check_visible = false;
        Buffer      buffer;
        Page        page;
        OffsetNumber offnum,
                    maxoff;

        /* Make sure we are interruptible. */
        CHECK_FOR_INTERRUPTS();

        /* Use the visibility map to decide whether to check this page. */
        if (all_frozen && VM_ALL_FROZEN(rel, blkno, &vmbuffer))
            check_frozen = true;
        if (all_visible && VM_ALL_VISIBLE(rel, blkno, &vmbuffer))
            check_visible = true;
        if (!check_visible && !check_frozen)
            continue;

        /* Read and lock the page. */
        buffer = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
                                    bstrategy);
        LockBuffer(buffer, BUFFER_LOCK_SHARE);

        page = BufferGetPage(buffer);
        maxoff = PageGetMaxOffsetNumber(page);

        /*
         * The visibility map bits might have changed while we were acquiring
         * the page lock.  Recheck to avoid returning spurious results.
         */
        if (check_frozen && !VM_ALL_FROZEN(rel, blkno, &vmbuffer))
            check_frozen = false;
        if (check_visible && !VM_ALL_VISIBLE(rel, blkno, &vmbuffer))
            check_visible = false;
        if (!check_visible && !check_frozen)
        {
            UnlockReleaseBuffer(buffer);
            continue;
        }

        /* Iterate over each tuple on the page. */
        for (offnum = FirstOffsetNumber;
             offnum <= maxoff;
             offnum = OffsetNumberNext(offnum))
        {
            HeapTupleData tuple;
            ItemId      itemid;

            itemid = PageGetItemId(page, offnum);

            /* Unused or redirect line pointers are of no interest. */
            if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid))
                continue;

            /* Dead line pointers are neither all-visible nor frozen. */
            if (ItemIdIsDead(itemid))
            {
                ItemPointerSet(&(tuple.t_self), blkno, offnum);
                record_corrupt_item(items, &tuple.t_self);
                continue;
            }

            /* Initialize a HeapTupleData structure for checks below. */
            ItemPointerSet(&(tuple.t_self), blkno, offnum);
            tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
            tuple.t_len = ItemIdGetLength(itemid);
            tuple.t_tableOid = relid;

            /*
             * If we're checking whether the page is all-visible, we expect
             * the tuple to be all-visible.
             */
            if (check_visible &&
                !tuple_all_visible(&tuple, OldestXmin, buffer))
            {
                TransactionId RecomputedOldestXmin;

                /*
                 * Time has passed since we computed OldestXmin, so it's
                 * possible that this tuple is all-visible in reality even
                 * though it doesn't appear so based on our
                 * previously-computed value.  Let's compute a new value so we
                 * can be certain whether there is a problem.
                 *
                 * From a concurrency point of view, it sort of sucks to
                 * retake ProcArrayLock here while we're holding the buffer
                 * exclusively locked, but it should be safe against
                 * deadlocks, because surely GetOldestXmin() should never take
                 * a buffer lock. And this shouldn't happen often, so it's
                 * worth being careful so as to avoid false positives.
                 */
                RecomputedOldestXmin = GetOldestXmin(NULL, PROCARRAY_FLAGS_VACUUM);

                if (!TransactionIdPrecedes(OldestXmin, RecomputedOldestXmin))
                    record_corrupt_item(items, &tuple.t_self);
                else
                {
                    OldestXmin = RecomputedOldestXmin;
                    if (!tuple_all_visible(&tuple, OldestXmin, buffer))
                        record_corrupt_item(items, &tuple.t_self);
                }
            }

            /*
             * If we're checking whether the page is all-frozen, we expect the
             * tuple to be in a state where it will never need freezing.
             */
            if (check_frozen)
            {
                if (heap_tuple_needs_eventual_freeze(tuple.t_data))
                    record_corrupt_item(items, &tuple.t_self);
            }
        }

        UnlockReleaseBuffer(buffer);
    }

    /* Clean up. */
    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);
    relation_close(rel, AccessShareLock);

    /*
     * Before returning, repurpose the fields to match caller's expectations.
     * next is now the next item that should be read (rather than written) and
     * count is now the number of items we wrote (rather than the number we
     * allocated).
     */
    items->count = items->next;
    items->next = 0;

    return items;
}

/*
 * Remember one corrupt item.
 */
static void
record_corrupt_item(corrupt_items *items, ItemPointer tid)
{
    /* enlarge output array if needed. */
    if (items->next >= items->count)
    {
        items->count *= 2;
        items->tids = repalloc(items->tids,
                               items->count * sizeof(ItemPointerData));
    }
    /* and add the new item */
    items->tids[items->next++] = *tid;
}

/*
 * Check whether a tuple is all-visible relative to a given OldestXmin value.
 * The buffer should contain the tuple and should be locked and pinned.
 */
static bool
tuple_all_visible(HeapTuple tup, TransactionId OldestXmin, Buffer buffer)
{
    HTSV_Result state;
    TransactionId xmin;

    state = HeapTupleSatisfiesVacuum(tup, OldestXmin, buffer);
    if (state != HEAPTUPLE_LIVE)
        return false;           /* all-visible implies live */

    /*
     * Neither lazy_scan_heap nor heap_page_is_all_visible will mark a page
     * all-visible unless every tuple is hinted committed. However, those hint
     * bits could be lost after a crash, so we can't be certain that they'll
     * be set here.  So just check the xmin.
     */

    xmin = HeapTupleHeaderGetXmin(tup->t_data);
    if (!TransactionIdPrecedes(xmin, OldestXmin))
        return false;           /* xmin not old enough for all to see */

    return true;
}

/*
 * check_relation_relkind - convenience routine to check that relation
 * is of the relkind supported by the callers
 */
static void
check_relation_relkind(Relation rel)
{
    if (rel->rd_rel->relkind != RELKIND_RELATION &&
        rel->rd_rel->relkind != RELKIND_MATVIEW &&
        rel->rd_rel->relkind != RELKIND_TOASTVALUE)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is not a table, materialized view, or TOAST table",
                        RelationGetRelationName(rel))));
}